WASHINGTON, March 19 (Yonhap) -- U.S. legislators appealed to the chief of the United Nations to press China to allow North Korean refugees under U.N. protection to leave the country, while accusing Beijing of using them as bargaining chips for the Summer Olympics.

A total of eight senators and representatives from Congress signed the letter addressed to U.N. Secretary-General Ban Ki-moon on Tuesday, expressing concerns about at least 17 North Koreans currently under the custody of the U.N. High Commissioner for Refugees (UNHCR).

"We understand that the PRC (People's Republic of China) is refusing them exit visas unless the UNHCR agrees not to process any more asylum seekers until after the Beijing Olympics," the letter said.

"We also understand that UNHCR has been unable to bring refugees into protection since July 2007 because of this policy." "We think you will agree that this practice by the PRC is unacceptable," the letter said. "The UNHCR needs your support and influence as the secretary-general of the United Nations to call upon Beijing to honor its international commitments and immediately provide these refugees exit visas so that they can safely be resettled." Thousands of North Koreans are believed to be hiding in China, which shares a border with their country, as they search for a way to resettle in a third country. But concerned about border security and an uncontrollable influx of refugees, Beijing has been cracking down on those whom it sees as illegal entrants, sometimes repatriating them back to Pyongyang, where they face harsh punishment, including execution.

The refugee issue has been further complicated as Beijing tightens security ahead of the Summer Olympics in August.